A magnetic tunnel junction (MTJ) device is in essence a variable resistor including two ferromagnetic layers, and a tunnel barrier sandwiched between them. The relative orientation of magnetization of the two ferromagnetic layers generates a difference in tunneling probability of electrons spin-polarized upon passing through the tunnel barrier, and this results in a change in resistance.
A tunnel barrier is generally made of a dielectric material, and must be very thin and have an extremely uniform thickness and composition. Nonuniformity in chemical composition or thickness of the tunnel barrier significantly degrades the device performance.
Ever since the invention of MTJ devices, the development of MTJ devices with a high TMR ratio at room temperature has become a hot industrial topic in order to realize spintronic applications such as a nonvolatile magnetic random access memory and a read head for a hard disk with an a real density higher than 100 Gbit/in2 (Moodera et al., Phys. Rev. Lett., 74 (1995), p 3273).
In the beginning, a high TMR ratio was achieved by a ferromagnetic electrode layer with high spin polarization and an amorphous AlOx tunnel barrier. The highest TMR ratio at room temperature, which was achieved with this configuration, is about 70%. After that, a spin filtering effect obtained by a single-crystal MgO tunnel barrier with the NaCl structure was proposed by theoretical calculation (Butler et al., Phys. Rev., B 63, (2001), p 054416). With this proposal, a TMR ratio as high as 6,000% at room temperature was predicted to be achieved.
This prediction is based on the fact that the crystal structure of single-crystal MgO has fourfold rotational symmetry, so an electronic state which exhibits a tunneling probability high enough to transmit MgO is only Δ1 with fourfold symmetry. Hence, conduction in a Δ1 band is predominant in an MTJ with a single-crystal Fe/MgO/Fe structure, but the Δ1 band in Fe is spin-polarized 100% at the Fermi level, so a sufficient tunneling probability cannot be obtained when an MTJ has antiparallel magnetization alignment. In other words, MgO has the effect of filtering spins in accordance with the magnetization alignment state.
This allows coherent tunneling, and, in turn, attains a giant TMR ratio. In order to achieve this giant TMR ratio, an experiment in which single-crystal Fe/MgO/CoFe is grown by molecular beam epitaxy was conducted, and the experimental result showed a TMR ratio of 180% at room temperature (Yuasa et al., Appl. Phys. Lett., 87 (2005), p 222508).
An MTJ device made of a combination of polycrystalline CoFe ferromagnetic electrodes and an MgO tunneling barrier has been reported to have a TMR ratio of 220% at room temperature (Parkin et al., Nat. Mater., 3 (2004), p 862). Moreover, an MTJ formed by depositing a combination of amorphous CoFeB and an MgO tunnel barrier on a silicon substrate, having thermally oxidized silicon deposited on it, by a magnetron sputtering method that is a practical deposition method has been reported to have a higher TMR ratio (Djayaprawira et al., Appl. Phys. Lett., 86 (2005), p 092502).
A great deal of effort has been put into forming an MTJ tunnel barrier which is very thin and has an extremely uniform thickness and composition. To form an oxide tunnel barrier, an important point is how to avoid oxidation of the surface of a ferromagnetic electrode layer under the tunnel barrier layer, and nonuniformity of the oxygen profile in the oxide tunnel barrier.
In general, methods of depositing tunnel barriers are classified into a method of directly depositing an oxide, and a method of depositing a metal and thereafter performing an oxidation process of the metal. RF-sputtering which uses an oxide target, or reactive sputtering in which a metal target is sputtered in an oxygen atmosphere exemplifies the method of direct deposition. In the method of depositing a metal and thereafter performing an oxidation process of the metal, natural oxidation, plasma oxidation, radical oxidation, or ozone oxidation exemplifies the oxidation process.